This is a factor of the education system within a country. for example in Kenya you need nursery (2 years), primary (8 years), high school(4 years), bachelors( 4 years) MSC(2 years) PhD( 3 years). That’s 23 years, then maybe you join nursery at 4 years. That’s 27 years. Maybe 25-27 years is a good guess
It depends on the education system. However as per the Kenyan curriculum, the youngest age to complete High school is 17, Then proceed to a 3yrs undergraduate program, Others do proceed directly to Phd program . So approximately the youngest age could be 23-24 yrs
It depends on whether you have completed the adequate coursework in order to apply for the PhD program that one is applying for.
Most programs (at least in the USA) require an undergraduate degree after high school. One graduates high school at 17-18 followed by 4 years of undergraduate education completing this at 21-22, then if one undertook a PhD for 5-6 yrs, this means one would graduate at 26-28 or so. Some PhD programs require masters degrees, so that will add/subtract some time from the timeline above. It is possible to graduate undergraduate college earlier if one has the appropriate credits, allowing some to graduate with their doctoral degrees at 23-25!